I don't get why Ace was so mad at Wallace. Wallace had nothing to do with Ace's father's death. He was a little kid, he wasn't even there. That whole plotline wasn't believable to me. He's not just going to turn on his longtime best friend like that.
I think it has more to do with the fact that Wallace snubbed him for his new relationship with his father. Ace was in serious danger, yet Wallace chose to be with his father over helping his best friend. The fact that Wallace's father killed Ace's was just salt in the wound.
We're your friends, we're not like the others man, really
He was mad at Wallace because Wallace told his girlfriend (Fiel's sister) that Fiel was still alive after Fiel was supposedly executed (by Fasto, then Midnight's 2nd-in-command) for the botched robbery earlier that day. Word reached Fiel that his being alive is no secret anymore as he heard that his sister knew he is still alive. And they knew that Midnight would soon learn about Fasto's betrayal -- and that's how the war properly started. The war was caused by Fasto's act of betrayal, but Wallace telling his girlfriend was the trigger that set it off.
Fiel's camp obviously blamed Ace for having "blabbed". Well, Ace had blabbed to Wallace, but he clearly told Wallace not to say a word about it to anyone. But Wallace did.
Therefore, Ace had a very valid reason to be annoyed at Wallace. Then the incident where Wallace couldn't help him to stay over at Wallace's dad's place, only increased Ace's anger (which is also justified.) Unfortunately, Wallace was not aware of the seriousness of these situations and the kind of hurt experienced by Ace.